1. Field of the Invention
The present invention relates to the manufacture of latex-, PVC- and plasticizer-free floor or wall coverings having chemically foamed and, if desired, structured foam layers.
2. Background
Textile floor or wall coverings having a foamed layer of a natural or synthetic latex- on the reverse side and PVC-plastic coverings are in widespread use today due to their versatile decorative application. These floor or wall coverings also are popular because of their simple installation and low cost.
The possibility of providing the coverings with soft, foamed backings which significantly contribute to footstep sound insulation and improve walking comfort, has ensured the continued use of such materials. Furthermore, the foaming permits the generation of surface structures either by foaming in suitable molds or on embossing rolls, or by partial chemical activation or inhibition of the foam formation. In the case of textile coverings, only mechanical foaming by the injection of air into the latex- compound is carried out commercially since the chemical foaming requires high temperatures. Such high temperatures are known to damage the textile nap.
In order to avoid joints during installation, floor coverings typically are manufactured in endless webs with a width of up to 4 or 5 meters, which considerably limits the possibilities from the point of view of both the material and the processing techniques.
Textile floor coverings generally are manufactured either by the tufting process or from needled non-woven fabrics, which are consolidated on the reverse side with a styrene-butadiene or other latex- compound. The fabrics then are coated with a mechanically foamed synthetic or natural latex- layer and fixed by drying. The textile layer comprises predominantly polyamide, polypropylene or polyester fibers, which can no longer be separated from the latex- layers. One disadvantage is that the composite layers cannot be recycled.
Plastic floor coverings have hitherto generally been manufactured from latex- dispersions or PVC- plastisols by a spreading process on a substrate of woven fabric or release paper, and subsequently cured. The plastisols consist of PVC- particles, plasticizers and conventional additives and fillers, which sinter together to produce a matrix upon drying under heat. By adding chemical foaming agents, the layer can additionally be thermally foamed. It typically is possible to achieve an additional structuring by applying blowing agent activators or deactivators to certain regions. It is naturally also possible, by applying several layers of different composition, to vary the properties of the floor covering to a wide degree.
Although polyamide and polyesters are excellent materials for textile coverings and PVC- is an excellent material from the point of view of its cost and its properties, ecological aspects, such as the possibility of recycling the products, avoidance of solvents and halogen-containing components must be accounted for. Therefore, there exists a need for processes for manufacturing coverings which are free of latex-, PVC- and plasticizer. For economic and technical reasons, however, it is necessary to retain the previous manufacturing widths and, if possible, also the existing manufacturing equipment. Furthermore the floor covering should also consist of different layers, one or more of which are chemically foamed and, if desired, partially structured by activating or deactivating the foaming process.
It is known from the so-called Furukawa process to prepare cross linked polyethylene foams by extruding polyethylene, azodicarbonamide as a blowing agent and dicumene peroxide as a crosslinking agent with the aid of an extruder, with downstream sheet die to produce a matrix in the form of a film or an unfoamed sheet. It is necessary for this extrusion process to take place at a temperature at which the polyethylene is liquid, but at which the cross linking agent has not yet decomposed. The free radical decomposition of the peroxide is initiated and the polyethylene cross linked with simultaneous chemical decomposition of the blowing agent and foaming of the matrix either after interim storage or by direct introduction of the matrix into a foaming oven. This process currently permits foams with densities between 30 and 175 kg/m.sup.3 and thicknesses between 5 and 15 mm to be manufactured. The width of these foams, however, is limited to about 2 m, since sheet dies of greater size cannot be manufactured. Therefore, economical manufacture of conventional floor coverings is not possible by this process.
It further is known that moldable foams can be manufactured from ethylene-vinyl acetate copolymers (EVA) or mixtures of EVA with polyethylene (PE) by mixing polymers, fillers, activators, foaming agents and, if required, cross linking agents at temperatures of from 90.degree. to 100.degree. C. At this temperature, the polymer is already soft or liquid, but the additives remain chemically stable. Subsequent to mixing at this temperature the mixture is granulated. The granules are subsequently introduced into molds, foamed by heating and removed from the mold after recooling. Relatively small, even complicated moldings can be manufactured satisfactorily in this manner. Examples of moldings which can be manufactured are shoe soles, balls, gaskets, mats, masks etc. The production of continuous webs with a width of 4 to 5 meters, as are necessary for floor coverings, is not possible by this process.
It further is known to produce unplasticized polyurethane foams by mechanical foaming of the components with the injection of air, but the foaming in this case cannot be chemically inhibited and thus no structure can be generated.